This invention relates to a method for treating polyimide surfaces for subsequent plating thereon. More particularly, this invention relates to a method for treating polyimide surfaces so as to improve the adhesion thereon of metal deposited during subsequent plating.
Printed circuit boards have become the dominant vehicle for mounting and interconnecting electronic components in order to manufacture a desired electronic circuit. The printed circuit board usually consists of a sheet of a dielectric, i.e., a nonconductive, substrate constructed from various filled or unfilled synthetic materials such as phenolics, glass-impregnated epoxies, and the like. The substrate is provided with a pattern of thin metal foil which functions as a conductive path on one or both sides. The paths or "traces" are usually formed of a conductive material such as copper, palladium, nickel or gold, and the like. The traces collectively define all of the electrical connections between components on the board, and are routed between appropriate locations.
The thermoplastic materials such as polyimides are particularly suitable for printed circuit board substrates because of their strength, heat resistance, dimensional stability, and easy moldability. However, polyimide substrates are not easily provided with a strongly adherent metal trace. The printed circuit, i.e., the plated metal conductive path, can be damaged or separated from the substrate during subsequent manufacturing steps or during use of the circuit board.
Several attempts have been made to increase the adhesion of the conductive metal traces to the polyimide substrates. Adhesion is generally measured as "peel strength", i.e., the force under controlled conditions required to peel the trace from the substrate. Some of these efforts focus on physically modifying the as-molded polyimide surface. For example, polyimide surfaces have been grit blasted to provide a roughened surface which anchors the subsequently-applied metals. Chemical swelling agents or penetrants have been used to swell the surface, and chemical etching agents have been used to remove portions of the surface.
While the methods described above which physically modify the polyimide surface do increase adhesion, they are often not entirely satisfactory for several reasons. Such techniques result in degradation of the molecules forming the polyimide surface, and may decrease both the tensile and impact strength of the substrate due to swelling and cracking of the entire substrate material, especially in those instances in which the polyimide contains fillers. Grit blasting requires cumbersome equipment and impedes the processing of printed circuit boards.
Dissatifaction with the above-described adhesion promotion techniques led to further attempts to improve adhesion between the polyimide surface and the metallization layer. These efforts focused on improving adhesion by forming a chemical bond between the polyimide and the plated metal.
For example, U.S. Pat. No. 4,775,449 to Dumas et al. describes an adhesion promotion treatment wherein a polyimide surface is treated with an adhesion-promoting compound characterized by the presence of an &gt;NO-- moiety, such as hydroxylamine. U.S. Pat. No. 4,842,946 to Foust et al. discloses an adhesion-promotion method which uses organic double bond-containing compounds such as thiourea to treat the polyimide surface.
The methods disclosed in U.S. Pat. No. 4,873,136 and copending, commonly-owned application Ser. No. 331,715, filed Apr. 03, 1989, both to Foust et al., in addition to the adhesion-promotion steps taken in the methods described in the patents to Dumas et al. and Foust et al. above, also subject the polyimide surface to an initial surface pretreatment which involves contact of the polyimide surface with a mild etching compound for removal of the surface layer, followed by contact with a basic solution which often includes short chain alcohols, and then contact with a solution of an alcohol and another mild etching compound. This pretreatment results in a clean, water-wettable surface which is particularly suitable for the subsequent adhesion-promoting treatment.
Experiments have shown that a heat treatment following electroless plating is necessary to develop chemical adhesion and that adhesion immediately following plating is probably mechanical in nature and due to the inherent surface roughness of most filled materials.
Although the above-described methods work well with filled polyimide surfaces, electroless plating on unfilled polyimide usually results in the blistering of the copper after a minimal time in the electroless plating bath. It is believed that the lack of mechanical adhesion on the smooth unfilled polymer is responsible. Processing of samples with a thin electroless layer is difficult and oxide removal after baking further reduces the deposit thickness, possibly causing resistance problems when electrolytic plating is attempted.
It is therefore desirable to provide a method for treating the surface of a polyimide which allows both filled and unfilled samples to be electroless plated in a consistent manner.
It is further desirable to provide a method for treating the surface of a polyimide which improves the adhesion between the polyimide surface and the plated metal.
Because of the problems associated with physical modification of the polyimide surface as described previously herein, it is desirable to modify the polyimide surface by means of a chemical technique. A chemical means for modifying the polyimide surface to achieve the objectives of the present invention would allow a single adhesion system to be used for all grades of polyimide and would provide an off-line process to resolve the problems associated with the use of unfilled polyimide substrates.
The objects listed above are achieved in the present invention by treating a polyimide surface with nitric acid or hydrochloric acid prior to the etching and adhesion-promoting steps previously described herein.
The treatment of a previously etched polyimide surface with nitric acid is described in U.S. Pat. No. 4,803,097 to Fraenkel et al., which discloses a method for treating a plastic surface in preparation for subsequent electroless plating. In the Fraenkel method, the surface of a plastic, which can be polyimide, is first etched by exposure to an ozone atmosphere, then treated with a conditioning agent containing a base and an alcohol so as to remove ozone/plastic degradation products from the ozonetreated plastic surface, and then treated with an oxidizing agent, preferably nitric acid, to further etch the plastic surface.
The treatment of a polymer with nitric acid or other nitrating agent is disclosed in U.S. Pat. No. 3,523,874 to Dey, which is directed to a process for making aromatic polymer surfaces susceptible to plating or coating with metals. In the Dey method, an aromatic polymer, typically a polystyrene or phenolic resin, is contacted with nitric acid or other suitable nitrating agent to form nitro radicals on the aromatic nuclei at the surface of the polymer. The preferred nitrating agent for use in the Dey process is disclosed to be a mixture of nitric acid and sulfuric acid. The nitrated surface is then contacted with a reducing agent to convert the nitro groups to amino groups. The resulting plastic surface is treated with a diazotization agent to convert the amino groups to N.sub.2 Cl groups and then contacted with an alcohol solution of hydroquinone to replace the N.sub.2 Cl groups with hydroquinone radicals. The resulting plastic can then be electroless plated.
German patent No. 3437084 discloses the etching of polyetherimide and other plastics for use in the manufacture of circuit boards. The plastic is etched with a sulfuric acid containing nitrate ions, the nitrate ions being provided preferably by nitric acid or its salts.
The treatment of polyimide resin with concentrated nitric acid is also disclosed in Japanese Patent No. 61231033, which is directed to a method of pulverising polyimide resin by dipping polyimide resin into concentrated nitric acid or a mixture of concentrated nitric acid and concentrated sulfuric acid and then grinding the treated resin to form powder.
The present invention is based on the discovery that treatment of an unetched polyimide resin with nitric acid or hydrochloric acid prior to the etching and adhesion-promoting treatments described hereinafter will result in increased adhesion between the plastic and the electrolessly deposited metal.